This invention relates to a method of measuring an electron beam landing error of a color display tube and particularly to a method of quantitatively measuring the tolerance of a color television tube to electron beam landing pertubations caused by temperature changes and/or stray magnetic fields.
The three beams of a color television picture tube must be adjusted in position for several reasons to enable a satisfactory picture to be reproduced on the viewing screen of the tube. Adjustment for color purity is required with all color picture tubes. The purity adjustment provides for the beams to land only on their respective color phosphor elements. Obviously, if the displayed picture lacks purity, the red beam, for example, might land on green or blue phosphors and result in a false color scene reproduction.
In a non-matrix type of color picture tube, the beam portion passing through an aperture of the shadow mask is smaller than the individual phosphor elements on the viewing screen so that when it is properly landed on the desired phosphor element it will not illuminate the adjacent different color elements. In a matrix-type of picture tube, in which dark guard bands separate adjacent different color phosphor elements, the beam portion passing through an aperture may be larger than the phosphor element and still result in color purity. In both types of picture tubes it is desirable to center the respective beams on their phosphor elements to minimize the possibility of a loss of purity if the beams are undesirably moved due to temperature changes of the picture tube or stray magnetic fields.
The distance between an edge of a beam and the adjacent different color phosphor is called purity tolerance. It is generally recognized that purity may be controlled at the center portion of a viewing screen by varying the position of two magnetized purity rings mounted for rotatable motion about the neck of the picture tube. Purity is adjusted at the edge regions of the picture tube by axial movement of the deflection yoke which moves the deflection center of the beams and hence controls their landing position at portions away from the center of the viewing screen.
Beam landing on the phosphor elements of a viewing screen may be observed by viewing the screen with the aid of a microscope. In a tube without guard bands between the phosphor elements, any purity errors, or clipping, can thereby be observed and the purity adjustment may then be made to correct any clipping condition. Such a procedure may be well suited for laboratory work; however, it is too time consuming for use on an assembly line where purity is normally adjusted. In a tube in which there are guard bands between the phosphor elements, the beam edge is hidden behind the guard band and errors in beam landing can be obscured.
On the assembly line, one common arrangement for setting purity is to bias off two of the three color beams and observe the color of the viewing screen. With only the red beam on, for example, ideally the viewing screen would display a pure red field. To accomplish this end, the purity rings around the neck of the tube are rotated to achieve a red field in the center portion of the viewing screen and the deflection yoke is moved to achieve a red field at all other portions of the viewing screen. As a practical matter, such adjustments are very subjective in nature as it is difficult to tell quickly whether the exact desired shade of red is displayed because various colors border the center region as the rings are adjusted and various colors appear in moving patterns around the viewing screen edge portions as the deflection yoke is moved. Utilizing this method of setting purity, even if a red field were obtained, there would be no way of knowing if the purity tolerance of the red beam from clipping blue and green phophor elements was equal. That is, whether the red beam was centered on the red phosphor elements. Thus, even though purity was obtained, it could represent only a marginal condition which could be upset by temperature changes or stray magnetic fields.
U.S. Pat. No. 3,723,801, issued to Oxenham, discloses a method of quantitatively measuring electron beam landing characteristics and adjusting color purity in a shadow mask cathode ray tube. In this method, a detector arrangement is placed in front of the screen, which detector is substantially insensitive to the colors with which the phosphor deposits luminesce which do not correspond to the electron beam activated. A first magnetic field is generated which causes the displacement of the electrons into a first direction. Two different values are successively given to the field in such a manner that the distance between the resultant spots of the electron beam on the screen is of the same order of the largest of the diameters of a phosphor dot or spot of the electron beam on the screen. A first direct current field of adjustable intensity is generated which causes the displacement of the electrons into the first direction. The intensity of this direct current field is adjusted in such a manner that the detector arrangement indicates substantially equal values for the two different values of the field. The measurement of electron beam landing error is then a function of the intensity of the direct current field which is required to cause the substantially equal values. Basically, this method permits the optimization of electron beam landing with respect only to its associated color phosphor elements. This method does not give an indication of purity tolerance, that is, the distance between an edge of a beam and the adjacent different color phosphor, unless it is assumed that the adjacent different color phosphors are perfectly symmetrical with respect to the measured color phosphor landing sites. As a practical matter, the adjacent different color phosphor elements are not perfectly symmetrical. Consequently, even though an electron beam is adjusted to land exactly in the center of its associated phosphor element, satisfactory color purity adjustment may not be attainable in the ultimate use environment due to temperature changes or stray magnetic fields which cause the electron beam to clip adjacent phosphor elements in asymmetrical screens.